Smoke detectors utilizing the so-called Tyndall effect are known which utilize as a light source a pulsing light-emitting diode. One such device is disclosed in my U.S. Pat. No. 3,946,241 issued Mar. 23, 1976. The use of a light-emitting diode which is energized by a very short pulse, as disclosed in said patent, has the desirable features of reducing substantially the possibility of false alarms and increasing the life of the light-emitting diode.
However, there is a need for a smoke detector that has a current drain low enough to allow it to be powered for a long period of time (in excess of one year) by a small battery, and that is sufficiently immune to false alarms to allow it to operate at a high sensitivity.